Pipes made from thermoplastic material are used, for example, as rigid pipes for sanitary purposes, for outdoor rainwater pipes, for water distribution and drains.
Pipes made from thermoplastic material are produced by an extrusion process, in a plant which draws the material in the plastic state, using a screw that rotates inside a cylinder, through a mould of suitable shape and dimensions.
The pipe production plant is known as extrusion line and it comprises a plurality of apparatuses, each designed for a specific function.
An apparatus, generally located at the end of the line, known as “cutter” is usually present in this system.
This apparatus is designed for cutting the pipe into pieces of pipe of precise and predetermined length.
This apparatus comprises a cutting unit installed on a movable carriage synchronized with the pipe and equipped with clamping means, designed for coupling with the pipe during the cutting operation.
With reference to the motion of the processing tool relative to the axis of the pipe, there are two different types of cutter apparatus: the shearing cutter apparatus and the planetary cutter apparatus.
The shearing cutter machines are characterised by a working motion of the cutting tool with direction of movement perpendicular to the axis of the pipe, whilst the planetary cutters are characterised by a working motion of the cutting tool with a circular movement relative to the axis of the pipe.
With reference to the cut, there are cutting techniques without removal of material and cutting techniques with removal of material.
The cutting techniques without removal of material can only be used for materials which are tough and with limited hardness, that is, materials characterised by high resistance to dynamic stresses and poor resistance to penetration of cutting tools. Examples of tough materials with limited hardness are the thermoplastics PE, PP and PB.
More specifically, these materials can be cut with cutting tools designed as blades with one or more cutting edges or with circular disk blades rotating freely about a respective axis or with guillotine blades.
More specifically, it should be noted that these cutting techniques can be used with pipes having relatively small wall thicknesses; on the other hand, with pipes having particularly large wall thicknesses, these cutting techniques are difficult to carry out because the cutting tool (generally in the shape of a circular disk) is subject to high levels of stress which favour deformation.
For materials with a particularly high hardness and a fragile-type mechanical behaviour the above-mentioned cutting techniques without removal of material are not practicable because these techniques would cause failure of the pipe during cutting (with possible damaging of the tool) and, in any case, the cut would be imprecise; in that case, the pipe is cut using cutting techniques with removal of material.
The cutting apparatus for these techniques comprises metal circular saws which are multi-serrated or have a surface coating of abrasive material.
It should be noted that the cutting by removing material generates large quantities of chippings which must be immediately removed from the cutting area to avoid malfunctioning of the cutting machine and/or other apparatuses nearby.
Moreover, these cuttings are harmful for the user and can electrostatically charge and adhere to the walls of the pipe making the subsequent processing of the pipe impracticable.
With particular types of materials which are particularly rich in mineral filler added to the base polymer, for example pipes made of amorphous material such as PVC-U, ABS and PMMA, there is the generation of dust which if not adequately removed from the cutting area can damage mechanical components of the apparatus and be harmful for the operators.
It should also be noted that cutting techniques with removal of material generate harmful vibrations which are transmitted to the machine components.
Other processing which may be performed on the pipe, in the extrusion line or also off line, is the chamfering of ends.
This processing is performed on the end of a piece of pipe and consists in making—by removing material—a chamfer on the end of a piece of pipe for allowing a sealed coupling with a cup or bell, that is, with the wide end of another piece of pipe.
It should be noted that this operation can be performed simultaneously with or after the cutting process.
In light of the above, there has been a long felt need for providing a method and an apparatus capable of processing a pipe (specifically, for chamfering) without removal of material (that is, without the generation of chippings and/or dust).
Even more specifically, the need is particularly felt for a method and an apparatus capable of chamfering the ends of pipes with particularly large wall thicknesses and/or pipes of particular high hardness and fragile mechanical behaviour.